The purpose of this series of studies is to clarify the importance of the gut permeability barrier in the development of heat injury and heat acclimation, and the mechanisms responsible (oxygen free radicals) for damaging the barrier. The authors propose that hyperthermia provokes intestinal ischemia and production of reactive oxygen species (ROS) which decrement barrier function, leading to the exit of LPS from the lumen of the gut into the circulation, in turn, generating cytokines and leading to hypotension. They further propose that heat shock proteins counteract the effect of ROS. They will also determine the location and time course of permeability dysfunction induced by heat. The specific experiments to be carried out address different aspects of the study. 1 Generation of ROS. They will test in vitro (2 strains in cell culture plus a strain transfected with MnSOD, monolayer conductance) and in vivo (51Cr-EDTA clearance, portal vein LPS levels, TNF, and HSP and inducible nitric oxide synthase levels, mannitol permeability) whether membrane function is altered by the elevations in ROS induced by heating, by dietary means and reduced by addition of an antioxidant to the system. 2. HSP protection. Cells expressing elevated HSP will also be tested in the above properties as well as the barrier function of acclimated rats.